This invention relates to devices for monitoring the electrical output of 3--phase ac generators, and more particularly to devices for detection of a missing ac phase. A generator output having a missing phase--that is to say only two of three phases are evident--can produce severe losses, poor operation and possibly damage to electrical loads. The need for monitoring 3--phase ac generators has therefore long been noted and missing phase detector devices have been employed. These missing phase detectors are well known in industry and are used in many applications for monitoring and protection of 3--phase loads and circuits.
Currently available missing phase detectors utilize solid-state components and integrated circuits to assist in speed and accuracy of detection. However, the available missing phase detectors still have a number of shortcomings which should be corrected. These are:
(a) The detectors tend to malfunction with motor loads when a missing phase occurs, due to a motor missing phase "fill-in" phenomenon; PA1 (b) The detectors are adversely affected by severely un-balanced loads, producing a false `missing phase` PA1 (c) The detectors are sensitive to brown-outs, often resulting in detector malfunction; and PA1 (d) The detectors are sensitive to frequency and voltage changes, and also to line distortion, producing errors.
Thus there exists a need for an improved missing phase detector that does not have the above listed problem areas.
The present invention device is presented in response to the foregoing need for an improved missing phase detector. The device comprises an isolation transformer, a three-phase rectifier and an analyzer circuit. The three-phase output of the equipment being monitored is connected to the primary windings of the isolation transformer which steps down the input voltages to low amplitudes induced in the secondary windings. The transformer secondary voltages are applied to a rectifier which produces a three phase rectified signal with respect to logic ground, and having a ripple wave shape. The ripple waveform is centered on a reference voltage and is passed to an analyzer circuit which examines the ripple wave signal, determines whether a missing phase is present and outputs a "missing phase" signal if one is detected. It does this by looking only at the ripple signal at its zero crossing points, without regard to peak levels.
Since the detector is not referenced to any fixed frequency or absolute voltage levels the detector is not sensitive to frequency or voltage changes or brownouts. Line distortion effects are not passed through and do not interfere with the analyzer. Similarly, unbalanced loads which affect voltage levels do not affect the analyzer portion of the detector.
An advantage of the invention is that unlike state of the art missing phase detectors, the invention detector will function accurately with power factor loads such as motors.
Another advantage of the invention is that it is highly reliable because of its small component count. An advantage accruing from the use of few components is the relatively low cost of the detector device.
Accordingly, it is a principal object of this invention to provide an improved missing phase detector device that is not sensitive to the monitored frequency or voltage levels or changes thereof.
Another object is to provide an improved missing phase detector device that is not adversely affected by monitored source line distortion, power brown-outs or sub-cyclic drop-outs.
Further objects and advantages of the invention will be apparent from studying the following portion of the specification, the claims and the attached drawings.